Power amps would do nothing for a powered sub like yours, the problem is how to integrate them. Even most integrated amps will do nothing for a sub in terms of bass management. You already have an avr with bass management, good room correction (Dirac) and you can add power amps to it as needed for various channels as it has a full set of pre-outs. Why get the 1140 instead of adding power amps? The 1140's amp section isn't all that interesting.
That sub you have, as was pointed out is rather weak, but really you can only do so much with a pair of 8" drivers, and while I like dual opposed drivers, not crazy about the "box". Better subs are better subs....and B&W isn't someone I'd look to for subs at all.
What are the rest of your speakers and why do you think you need more power for them?
As to used gear, depends. Are you in the UK?